1. Field of the Invention
The present invention relates to a transmission/reception apparatus, and more particularly, to a transmission/reception apparatus that assigns a spread transmission signal for a subcarrier for each spread signal to perform frequency division multiplexing in a mobile communication, and thereby performs radio communications in a combination system of OFDM/TDD and CDMA, and a transmission diversity method for such an apparatus.
2. Description of the Related Art
In a CDMA communication, interference between spreading codes takes place in multipath environment, and an error rate characteristic deteriorates. Meanwhile, as a communication system resistant to the interference between spreading codes, there is known an OFDM communication using guard intervals. Therefore as a next generation system, attention is drawn to an OFDM-CDMA system radio communication which provides multicarrier in the CDMA communication, assigns a subcarrier signal for each chip, and thereby performs frequency division multiplexing to transmit signals.
In the OFDM-CDMA communication, a plurality of signals are spread with different non-correlative spreading codes, and one spread signal is assigned one subcarrier. When the spreading codes are completely orthogonal, it is possible to completely remove signals except required signals by despreading processing in receiving the signals even if the signals are multiplexed greatly.
The following explains a conventional OFDM-CDMA transmission/reception apparatus using FIGS. 1 and 2. FIG. 1 is a block diagram illustrating a schematic configuration of the conventional transmission/reception apparatus, and FIG. 2 is a schematic diagram illustrating one example of subcarrier assignments in a conventional OFDM-CDMA communication.
In a transmission system in FIG. 1, spreading section 1 multiplies transmission signals 1 to n respectively by spreading codes 1 to n to perform spreading processing. Herein a spreading factor is assumed to be k.
Addition section 2 adds spread transmission signals, and Serial-Parallel; S/P converter 3 converts single-sequence signals into plural-sequence signals. Herein added spread transmission signals are divided for each spread signal, and spread transmission signals 1 to n are disassembled into the first to kth chips for each spread signal (chip).
IFFT processing section 4 performs Inverse Fourier Transform processing on the plural-sequence signals, and at this point, assigns one subcarrier for one chip data signal sequence to process frequency division multiplexing.
That is, the number of subcarriers matches with the spreading factor, and herein is k. In addition, it is assumed that subcarrier 1 is assigned for first chips of transmission signals 1 to n, and subcarrier k is assigned for kth chips of transmission signals 1 to n. In other words, chip data sequences are subjected to the frequency division multiplexing. FIG. 2 illustrates this aspect. Antenna 5 performs transmission and reception of radio signals.
In a reception system, FFT processing section 6 performs Fourier. Transform processing on a received signal to obtain each subcarrier signal (chip data signal sequence). Compensation sections 7 are provided for each subcarrier, and perform compensation processing such as phase compensation on respective subcarrier received signals.
Parallel-Serial; P/S converter 8 converts the plural-sequence signals into the single-sequence signals, specifically rearranges each subcarrier signal for one chip, outputs at time t1 the first chip of a signal obtained by multiplexing spread transmitted signals 1 to n, outputs at time t2 the second chip of the signal obtained by multiplexing spread transmitted signals 1 to n, and then sequentially outputs at time tk the kth chip of the signal obtained by multiplexing spread transmitted signals 1 to n.
Despreading section 9 multiplies received signals, each of which is converted into the single-sequence signal, by respective spreading codes 1 to n, and obtains only signals spread with the codes to perform despreading.
However the conventional transmission/reception apparatus has the following problem. That is, in the multipath environment, each subcarrier signal is affected by fading variation independently, causing a case that received amplitudes are different between subcarrier signals as illustrated in FIG. 3.
In the OFDM-CDMA communication, since one subcarrier is assigned for each chip assignment position of each spread transmission signal, i.e., one subcarrier is assigned for one chip, to perform the frequency division multiplexing, a deviation in received amplitude of each subcarrier signal directly becomes a deviation in received amplitude of the spread signal, and as a result, the orthogonality deteriorates.
That is, spreading codes are selected so that each spreading code is orthogonalized to each other. However there is the assumption that amplitudes of the spreading codes are constant, and therefore the orthogonality deteriorates when divinations are generated in received amplitudes of spreading codes.
For example, the correlation of spread signal sequence RX [1, −1, 1, 1] with spreading code sequence TX [−1, −1, 1, −1] is as follows:RX·TX=[1, −1, 1, 1]·[−1, −1, 1, −1]=1×(−1)+(−1)×(−1)+1×1+1×(−1)=0 where the orthogonality is confirmed.Herein assume that amplitude divination is generated in spreading code sequence RX, and that RX becomes RX′[3, −0.1, 0.2, 1]. In this case, the correlation is as follows:RX′·TX=[3, −0.1, 0.2, 1]·[−1, −1, 1, −1]=3×(−1)+(−0.1)×(−1)+0.2×1×(−1)==−3.7 where the orthogonality deteriorates.
Thus in the multipath environment, when the orthogonality between spreading codes deteriorates, other signal components remain as noise components corresponding to deterioration of the orthogonality, and thereby the error rate characteristic deteriorates. Since the noise components are increased as the number of multiplexed signals is increased, the deterioration of the error rate characteristic is proportional to the number of multiplexed signals, and as a result, the extent of deterioration is increased as the number of multiplexed signals is increased.
Generally in the radio communication system, since the error rate is controlled to be below a predetermined level, the number of signals to be multiplexed is decreased to prevent the deterioration of the error rate characteristic, and thereby the transmission capacity is decreased.